When using get_property(), you need to make sure the property you are using is not set as write-only. An example of such a property is GtkContainer::child, which isn't readable. The warning when trying to read it is something like:

  (:2640): Gtk-WARNING **: ../../gtk/gtkcontainer.c:874: invalid property
           id 3 for "child" of type `GParamObject' in `GtkButton'

Pretty much all instances of write-only properties in GTK are ones that set some other property, but take a different format. For instance, setting the "markup" property on a gtk.CellRendererText object is equivalent to setting the "text" and "attributes" properties to appropriate values, however "markup" is not readable.

The GtkContainer "child" property looks more like a convenience. Setting the property adds the widget to the container. However, GtkContainer can have multiple children, so it isn't clear what should be returned when reading it.